There is experimental evidence that platelets, by interacting with the arterial wall, may be important in the initiation of atherosclerosis. Our preliminary observations indicate that pigs with impaired platelet adhesiveness in the form of von Willebrand's disease, are resistant to the development of spontaneous atherosclerosis. We are studying this in more detail experimentally. 1. To evaluate by a prospective study whether pigs with von Willebrand's disease (vWd) and normal pigs treated with platelet inhibitor drugs are less prone to the development of spontaneous atherosclerosis than normal control pigs eating a normal diet. 2. To determine whether or not pigs with von Willebrand's disease and normal pigs treated with platelet inhibitor drugs are less prone than normal control pigs to the development of atherosclerosis induced by a high cholesterol diet. 3. To determine whether or not pigs with von Willebrand's disease and normal pigs treated with platelet inhibitor drugs are less prone than normal control pigs to the development of atherosclerosis induced by chronic endothelial injury. These biological experiments may help in elucidating the role of platelets in the development of atherosclerosis in pigs and the potential of platelet inhibitors in its prophylaxis.